Research Highlights

Pure and Locked Spin-Valley Diffusion Current: The Road to Quantum Computing

2018-07-20 562


Quantum computing, or computing using quantum-mechanical phenomena such as superposition and entanglement, may one day be the key to solving complex problems (perhaps even “the answer to life, the universe and everything”) that even the fastest supercomputers in existence cannot begin to tackle. Classical computing uses a bit, or a single piece of information that can exist in one of two states: 1 or 0. In contrast, quantum computing uses quantum bits (qubits) which can exist in not just 1 or 0 but also as a superposition of these values. Simply put, quantum computing has the potential to store exponentially larger amounts of information using less energy than classical computing.

As quantum computing holds immense real world implications to critical issues such as cybersecurity, researchers throughout the world are in an arms race of sorts to harness its powers. To many scientists, spintronics (where the spin of an electron and its associated magnetic moment is exploited) and valleytronics (where information is coded based on ‘the wavelike motion of electrons moving through certain two-dimensional semiconductors’) are of particular interest because they offer tremendous advantages in data storage and processing when compared to the electrical charges used in classical computing. Consequently, transition metal dichalcogenide (TMDC) materials with their distinctive structures and strong spin-orbital interactions are extremely promising for spintronics and valleytronics. However, existing methods could not efficiently generate spin-valley current—a crucial requirement for research and application purposes.

Collaborative research conducted by Professor Jonghwan Kim from the Department of Materials Science and Engineering at Pohang University of Science and Technology and Professor Feng Wang from the Department of Physics at University of California at Berkeley has successfully demonstrated the efficient generation of a pure and locked spin-valley diffusion current in a TMDC heterostructure without any driving electric field. This achievement was published in the world-renowned Science.

The research team encapsulated a tungsten disulfide (WS2)-tungsten diselenide (WSe2) heterostructure in two hexagonal boron nitride flakes with a few-layer graphene back gate. This stack was then transferred onto a SiO2-Si substrate to complete the heterostructure device. The team was then able to image the propagation of the valley current in real time and space—both of which are controllable through electrostatic gating. Furthermore, as the device does not require an associated charge current, the pure valley diffusion current coupled with a pure spin diffusion current paves the way for new types of highly efficient spin and valley devices.

Professor Kim expressed his anticipation that this elegant demonstration of ultra-long spin-valley lifetimes and diffusion lengths will lead to TMDC heterostructures providing exciting opportunities for the future of spintronic and valleytronic devices.

The Office of Basic Energy Sciences, Materials Sciences, and Engineering Division of the U.S. Department of Energy, the NSF EFRI program, the Elemental Strategy Initiative conducted by MEXT, the JSPS KAKENHI grant, the NSF DMR CAREER award, and the Deutsche Forschungsgemeinschaft supported this research.